Many wireless communications networks utilize base stations each positioned to cover a corresponding “cell” within which a wireless communications device may operate. As used herein, the term “wireless communications device” generally refers to any device being suitable for communicating in a wireless fashion using a wireless communications network. Examples of such devices include, but are not limited to: wireless telephones, i.e., handsets, and mobile computing devices including such functionality, such as portable Personal Computers (PCs) and Personal Digital Assistants (PDAs). Further, each cell may be divided into “sectors” as is also well understood.
A cell typically covers a limited geographic area and routes communications between wireless communications devices physically located within the limited geographic area and a telecommunications network, such as a Public Switched Telephone Network (PSTN). When a wireless communications device physically moves from one cell and/or sector to another, a “handoff” may be performed to coordinate operation of the wireless communications device with the new cell and/or sector. Generally, a handoff typically involves negotiating instructions between the moving wireless communications device and one or more base stations and/or mobile switching centers corresponding to the location of the wireless communications device. This serves to keep a communications session, such as a phone call, active as the wireless communications device traverses from one cell and/or sector to another. A handoff may be triggered, for example, when the wireless communications device detects a pilot signal from a different base station that exceeds a given threshold.
Referring now to FIG. 1, there is shown an illustration of a handoff threshold example for a CDMA system as a plot of pilot signal strength versus time. At time (1), as the pilot signal strength 10 exceeds a threshold T_ADD, the mobile communications device sends a Pilot Strength Measurement Message and transfer the pilot to a Candidate Set 20. At time (2), the corresponding base station sends a Handoff Direction Message. At time (3), the mobile communications device transfers the pilot to the Active Set 30 and sends a Handoff Completion Message. At time (4), as the pilot signal strength 10 drops below a threshold T_DROP, the mobile communications device starts a handoff drop timer. At time (5), the handoff drop timer expires, and the mobile communications device sends a Pilot Strength Measurement Message. At time (6), the corresponding base station sends a Handoff Direction Message. At time (7), the mobile communications device moves the pilot from the Active Set 30 to the Neighbor Set 40 and sends a Handoff Completion Message. This technique is well understood by those possessing an ordinary skill in the pertinent art.
In a wireless communications network utilizing a Code Division Multiple Access (CDMA) system, a handoff from one cell and/or sector to another involves estimating the timing of a received signal. Wireless communications devices communicating via CDMA systems may experience delays in reception of signals due to the speed of light, reflections, and mobility of the wireless communications devices. When a wireless communications device moves from a first cell to a second cell, a handoff is performed to assign new system resources associated with the second cell. CDMA systems may employ “Search Windows” of a settable length over which to search for the potentially delayed signals. A Search Window is typically set around spreading (or PseudoRandom—PN) sequence phase offsets where geographically adjacent base stations are expected to be transmitting. If a utilized Search Window is set too wide, the searching process can temporally delay handoffs, reducing performance of the wireless communications devices and capacity of the network system. Further, if the Search Window is set too narrow, wireless communications devices may not acquire delayed signals, which may lead to poor performance and dropped communications, such as calls, due to one or more missed handoff opportunities.
Conventionally, two methods for setting Search Windows may be employed: default settings may be used based upon general manufacturer recommendations; or vehicles with test equipment may be moved throughout the cells and sectors and used to collect timing delay information to calculate Search Window settings.
The first method may prove satisfactory for many handoff scenarios, but typically does not provide for even nearly-optimal settings. The second method can provide improved performance over the first method, but, is often expensive, time-consuming, and typically requires large areas to be covered in the data collection process—further driving up costs in both dollars and time.